The present invention relates generally to endoluminal prostheses or xe2x80x9cstents,xe2x80x9d and more particularly to stents including a micro-porous mesh structure supported by an integral or separate strut structure, and to methods of making and deploying such stents.
Tubular prostheses or xe2x80x9cstentsxe2x80x9d are often implanted within blood vessels, for example, within the coronary and carotid arteries, for treating atherosclerotic disease that may involve one or more stenoses. Stents generally have a tubular shape capable of assuming a radially contracted condition to facilitate introduction into a patient""s vasculature, and an enlarged condition for engaging the vessel wall at a treatment location.
Plastically deformable stents have been suggested that are initially provided in their contracted condition, and placed over a balloon on an angioplasty catheter. At the treatment location, the balloon is inflated to plastically deform the stent until it is expanded to its enlarged condition.
Self-expanding stents have also been suggested that are biased to assume an enlarged condition but may be radially compressed to a contracted condition. The stent may be mounted to a delivery device and constrained in the contracted condition during delivery, for example, by an overlying sheath. At the treatment location, the stent may be released, for example, by retracting the sheath, the stent automatically resuming its enlarged condition to engage the vessel wall.
In addition to tubular stents, coiled-sheet stents have been suggested that include a flat sheet rolled into a spiral or helical shape having overlapping inner and outer longitudinal sections. Such stents generally have a lattice-like structure formed in the sheet and a plurality of fingers or teeth along the inner longitudinal section for engaging openings in the lattice. Once deployed at a treatment location, the fingers may engage openings in the lattice to lock the stent in the enlarged condition.
One of the problems with many stent structures, whether balloon-expandable or self-expanding, is that they substantially expose the underlying wall of the treatment location. For example, helical wire stent structures generally have substantial gaps between adjacent turns of the wire. Multi-cellular stent structures, which may include a series of slotted or zig-zag-shaped cells, create large spaces within and/or between the cells, particularly as they expand to their enlarged condition. The lattice structure of coiled-sheet stents generally also includes relatively large openings.
Thus, despite holding the wall of the treatment location generally open, the openings or gaps in these stents may substantially expose the bloodstream to plaque, tissue prolapse, or other embolic material attached to the wall of the vessel. This embolic material may be inadvertently released during or after deployment of the stent, and then travel downstream where it may cause substantial harm, particularly if it reaches a patient""s neurovasculature.
One proposed solution to address the issue of embolic containment is to cover a conventional stent with a fabric or polymer-type material. This solution has met with limited success, however, because of the propensity to form false lumens due to poor apposition of the covering and the vessel wall.
Accordingly, it is believed that a stent capable of supporting the wall of a blood vessel being treated, while substantially minimizing exposure of embolic material to the bloodstream, would be considered useful.
The present invention is directed to an endoluminal prosthesis or xe2x80x9cstentxe2x80x9d including a micro-porous mesh structure and a support structure, and to methods of making and implanting such stents. In accordance with one aspect of the present invention, a micro-porous mesh structure for supporting a wall of a body passage is provided that includes a generally tubular body having a contracted condition for facilitating delivery into the body passage, and an enlarged condition for engaging the wall of the body passage, the tubular body having a preferred wall thickness of not more than about 25 micrometers (0.001 inch). A plurality of openings are provided in the tubular body defining a mesh pattern therein, each opening preferably having a maximum dimension of not more than about 200 micrometers (0.008 inch).
In a preferred form, the tubular body is a coiled-sheet having overlapping inner and outer sections formed from a material, such as Nitinol, exhibiting temperature-activated shape memory properties. A plurality of struts may be formed integrally onto the tubular body and spaced along a length of the tubular body for supporting the tubular body against the wall of the body passage, having, for example, a thickness of about 100-150 micrometers (0.004-0.006 inch).
In accordance with another aspect of the present invention, a prosthesis for supporting a wall of a body passage is provided that includes a tubular element, such as the micro-porous mesh structure described above, and a separate support element including a plurality of struts for engaging an interior surface of the tubular element. The support element is preferably biased to an enlarged condition at body temperature for substantially securing the tubular element against the wall of the body passage in the enlarged condition. The support element may be any of a variety of known stent structures, such as a coiled-sheet stent, preferably formed from a material, such as Nitinol, exhibiting temperature-activated shape memory properties.
In accordance with yet another aspect of the present invention, a method for making a prosthesis for supporting a wall of a body passage includes providing a sheet formed from a shape memory alloy, preferably having a transition temperature between a substantially ambient temperature and body temperature, the sheet having a wall thickness of not more than about 25 micrometers (0.001 inch). A mesh pattern is formed in the sheet that includes a plurality of micro-porous openings, and the sheet is formed into a generally tubular body, for example, by rolling it into a coiled-sheet having overlapping inner and outer sections.
The tubular body may be heat treated at a first temperature substantially higher than the transition temperature to program an expanded condition for engaging the wall of the body passage into the shape memory material. The tubular body may then be cooled to a second temperature below the transition temperature, and compressed into a contracted condition for facilitating delivery into the body passage.
In one preferred form, the sheet has an initial wall thickness greater than about 25 micrometers (0.001 inch) and not more than about 150 micrometers (0.006 inch). Portions of the sheet are removed to provide a plurality of struts having a thickness similar to the initial wall thickness separating thin-walled regions having a final wall thickness of not more than about 25 micrometers (0.001 inch). In another preferred form, a separate strut element may be provided for supporting the tubular body that may be attached to the tubular body.
In accordance with still another aspect of the present invention, a method for supporting a wall of a predetermined location within a body passage incorporates a prosthesis including a micro-porous tubular element and a separate support element, such as that described above. The tubular and support elements are placed in contracted conditions on a distal region of a delivery device, such as a catheter. The distal region of the delivery device is advanced endoluminally within the body passage to the predetermined location. The tubular element is deployed at the predetermined location, and the support element is expanded to an enlarged condition to engage an interior surface of the tubular element, thereby substantially securing the tubular element against the wall of the predetermined location.
Preferably, the support element is biased to expand to its enlarged condition at body temperature such that upon deployment from the delivery device, the support element automatically expands to engage the interior surface of the tubular element. The tubular element may be expanded to an enlarged condition as the support element expands to its enlarged condition, or alternatively, the tubular element may also be biased to expand to an enlarged condition at body temperature such that the tubular element automatically expands to its enlarged condition to conform to the wall of the predetermined location upon deployment from the delivery device.
Thus, a prosthesis in accordance with the present invention may be used to treat a stenosis within a blood vessel, such as within the carotid, coronary or cerebral arteries. The tubular element may substantially trap embolic material against the wall of the vessel once secured by the support element, while the micro-porous mesh pattern facilitates endothelial growth, thereby substantially reducing the risk of releasing embolic material into the bloodstream. Also, the provision of a micro-porous mesh structure and a separate support structure may allow the support structure to slide along the surface of the tubular element without significantly disturbing the underlying diseased vessel wall.
In addition, in a preferred embodiment, the micro-porous mesh structure is biased to its enlarged condition such that the tubular body may tend to expand out to engage the vessel wall. Thus, the micro-porous mesh structure may contact the vessel wall at points between the struts of the support structure, thereby minimizing the creation of false lumens or other gaps between the micro-porous mesh structure and the vessel wall.
Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.